1. Introduction
This invention relates to catalysts for electroless metal deposition. More particularly, this invention relates to a catalytic process especially useful for catalyzing substrates having a metal such as copper over at least a portion of its surface and catalyst materials useful in said process. The catalysts of the invention are useful for multilayer circuit fabrication and for the manufacture of other industrial products.
2. Description of the Prior Art
Electroless metal deposition is the deposition of metal over a substrate in the absence of an external electric current. The procedure is especially useful for metal plating substrates where at least a portion of the substrate is of a dielectric material. Electroless metal deposition processes and catalytic materials are known in the art and described in numerous publications.
An early patent disclosing an electroless plating catalyst used in large volume is U.S. Pat. No. 3,011,920, incorporated herein by reference. In accordance with this patent, an electroless plating catalyst is formed by reduction of palladium ions with stannous ions to form a tin-palladium colloid. A substrate is plated by a step of immersion of the substrate in the colloidal catalyst whereby colloidal particles absorb on the surface of the substrate. The substrate is then subjected to a step of acceleration and metal plated by immersion of the substrate in a metal plating solution that contains dissolved metal ions of a metal to be plated. Contact of the metal ions with the colloidal palladium catalyst in the presence of a reducing agent results in the electroless metal deposition of the metal over the substrate. An improved catalytic process is disclosed in published European Patent Application, Publication No. 0 510 711, incorporated herein by reference. In accordance with preferred procedures of this application, a process for metallization comprises the steps of formation of a ligating layer over a substrate having terminus groups capable of bonding with a catalyst precursor. Preferably, the terminus groups comprise a metal ion binding or ligating group, and the layer is a self-assembled film having terminus ligating groups. Following formation of the ligating layer, the layer is contacted with a catalyst precursor solution such as a solution believed to be a palladium-chloro complex. The complex bonds with the ligating groups of the ligating layer. Subsequent contact of the layer with an electroless metal plating catalyst containing a suitable reducing agent results in selective electroless metal deposition of metal onto the ligating layer.
A disadvantage to the above process involves contact of the chloro complex with a substrate having a metallic portion such as a printed circuit board substrate. In practice, it has been found that contact of the metallic portion of the substrate with the chloro complex results in formation of a displacement coating over the metallic portion of the surface. The displacement coating is poorly adherent to the underlying metal and therefore, subsequently applied electroless metal over the displacement coating is poorly bonded to the underlying metal surface and has a tendency to flake off during use. In addition to the formation of a displacement coating, the chloro complex used in the process of the EPO application has a short shelf life. In practice, it has been found that upon standing for short periods of time, e.g., 24 hours, the complex converts to an unstable colloid, probably by chain-length extension, and the so formed unstable colloid is unsuitable for use as an electroless plating catalyst.